Why are we implementing, for example ICloneable or IDisposable. I'm not asking what ICloneable or IDisposable do, but I want to learn what's a good reason to implement these interfaces rather than just writing a method which disposes or clones our objects?
Using interfaces keeps the use of those pieces of functionality consistent. That way, when another class wants to / needs to use your class, it can act on it as a cloneable, disposable object without worrying about your particular implementation details.
By implementing a well known interface, you can have polymorphism, which enable you to write generic code that can act on any instance of a class that implementes a given interface.
You can check the Wikipedia article on polymorphism for more.
Because you may want the same code to operate on instances of different classes. For example, a form cleanup routine wants to iterate over all components and dispose them. In order for it to do this, it needs to refer to the components through a type: either a common base class or an interface. Single inheritance means that a common base class isn't always feasible (suppose my form has both a FileStream and a Button -- what common base class could they have that the cleanup routine would access them through?); hence interfaces.
Adding to more interface wisdom, interfaces can be used as contracts between consumer code and service code. Instead of saying this is object that we will be dealing with, with interfaces the agreement is, this is how the object i return looks. In code, it could be something like:
service code
public IEnumerable<int> GetNumbers(){ return new []{1,2,3,4,5}; }
client code :
var result = serviceProxy.GetNumbers();
Here the service code can change implementation to return any class that satisfies IEnumerable without breaking client code.
Besides all this you have got other applications like IoC DI, Unit Testing, Object Mocking. All this reaping benefits of polymorphic goodness.
Among other reasons, read about the using block.
Phoebus gave a great answer, but to add a bit more.
It forces you to write the methods that are expected for cloning or making something that can be disposed. If you had to write just the method, would you always include all the necessary methods, or would the signature always be the same?
If there is a new framework, and they add something, to help ensure that the operations are done correctly, by having interfaces then it forces you to implement any new changes, as your application will no longer compile.
In summary the Interface separates the implementation and defines the structure, and this concept is very useful in cases where you need the implementation to be interchangeable. Apart from that an interface is very useful when the implementation changes frequently.
Interface can be used to define a generic template and then one or more abstract classes to define partial implementations of the interface.
Interfaces just specify the method declaration (implicitly public and abstract) and can contain properties (which are also implicitly public and abstract). Interface definition begins with the keyword interface. An interface like that of an abstract class cannot be instantiated.
An interface separates what's done from how it's done. Clients deal with the interface type, without having to know what the implementation class is, because they can rely on the contract it enforces.
This is important for situations that generate dynamic proxies. Clients need not know that they're dealing with a proxy, which gives you the freedom to inject whatever behavior you need. It's a common technique for aspect-oriented programming.
Related
I am developing a set of classes that implement a common interface. A consumer of my library shall expect each of these classes to implement a certain set of static functions. Is there anyway that I can decorate these class so that the compiler will catch the case where one of the functions is not implemented.
I know it will eventually be caught when building the consuming code. And I also know how to get around this problem using a kind of factory class.
Just curious to know if there is any syntax/attributes out there for requiring static functions on a class.
Ed Removed the word 'interface' to avoid confusion.
No, there is no language support for this in C#. There are two workarounds that I can think of immediately:
use reflection at runtime; crossed fingers and hope...
use a singleton / default-instance / similar to implement an interface that declares the methods
(update)
Actually, as long as you have unit-testing, the first option isn't actually as bad as you might think if (like me) you come from a strict "static typing" background. The fact is; it works fine in dynamic languages. And indeed, this is exactly how my generic operators code works - it hopes you have the static operators. At runtime, if you don't, it will laugh at you in a suitably mocking tone... but it can't check at compile-time.
No. Basically it sounds like you're after a sort of "static polymorphism". That doesn't exist in C#, although I've suggested a sort of "static interface" notion which could be useful in terms of generics.
One thing you could do is write a simple unit test to verify that all of the types in a particular assembly obey your rules. If other developers will also be implementing the interface, you could put that test code into some common place so that everyone implementing the interface can easily test their own assemblies.
This is a great question and one that I've encountered in my projects.
Some people hold that interfaces and abstract classes exist for polymorphism only, not for forcing types to implement certain methods. Personally, I consider polymorphism a primary use case, and forced implementation a secondary. I do use the forced implementation technique fairly often. Typically, it appears in framework code implementing a template pattern. The base/template class encapsulates some complex idea, and subclasses provide numerous variations by implementing the abstract methods. One pragmatic benefit is that the abstract methods provide guidance to other developers implementing the subclasses. Visual Studio even has the ability to stub the methods out for you. This is especially helpful when a maintenance developer needs to add a new subclass months or years later.
The downside is that there is no specific support for some of these template scenarios in C#. Static methods are one. Another one is constructors; ideally, ISerializable should force the developer to implement the protected serialization constructor.
The easiest approach probably is (as suggested earlier) to use an automated test to check that the static method is implemented on the desired types. Another viable idea already mentioned is to implement a static analysis rule.
A third option is to use an Aspect-Oriented Programming framework such as PostSharp. PostSharp supports compile-time validation of aspects. You can write .NET code that reflects over the assembly at compile time, generating arbitrary warnings and errors. Usually, you do this to validate that an aspect usage is appropriate, but I don't see why you couldn't use it for validating template rules as well.
Unfortunately, no, there's nothing like this built into the language.
While there is no language support for this, you could use a static analysis tool to enforce it. For example, you could write a custom rule for FxCop that detects an attribute or interface implementation on a class and then checks for the existence of certain static methods.
The singleton pattern does not help in all cases. My example is from an actual project of mine. It is not contrived.
I have a class (let's call it "Widget") that inherits from a class in a third-party ORM. If I instantiate a Widget object (therefore creating a row in the db) just to make sure my static methods are declared, I'm making a bigger mess than the one I'm trying to clean up.
If I create this extra object in the data store, I've got to hide it from users, calculations, etc.
I use interfaces in C# to make sure that I implement common features in a set of classes.
Some of the methods that implement these features require instance data to run. I code these methods as instance methods, and use a C# interface to make sure they exist in the class.
Some of these methods do not require instance data, so they are static methods. If I could declare interfaces with static methods, the compiler could check whether or not these methods exist in the class that says it implements the interface.
No, there would be no point in this feature. Interfaces are basically a scaled down form of multiple inheritance. They tell the compiler how to set up the virtual function table so that non-static virtual methods can be called properly in descendant classes. Static methods can't be virtual, hence, there's no point in using interfaces for them.
The approach that gets you closer to what you need is a singleton, as Marc Gravell suggested.
Interfaces, among other things, let you provide some level of abstraction to your classes so you can use a given API regardless of the type that implements it. However, since you DO need to know the type of a static class in order to use it, why would you want to enforce that class to implement a set of functions?
Maybe you could use a custom attribute like [ImplementsXXXInterface] and provide some run time checking to ensure that classes with this attribute actually implement the interface you need?
If you're just after getting those compiler errors, consider this setup:
Define the methods in an interface.
Declare the methods with abstract.
Implement the public static methods, and have the abstract method overrides simply call the static methods.
It's a little bit of extra code, but you'll know when someone isn't implementing a required method.
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Interface vs Abstract Class (general OO)
I can see their advantage in coordination of a developing team, or code that might be further developed by others.
But if not, is there a reason to use them at all? What would happen if I omit them?
Abstract – I'll be able to instantiate it. No problem. If it doesn't make sense – I won't.
Interface – I have that functionality declared in all classes deriving from it anyway.
Note: I'm not asking what they are. I'm asking whether they're helpful for anything but coordination.
Both are what I call contracts and can be used in the following fashion by an individual developer:
Abstract
Allows for polymophism of differing derived implementations.
Allows one to create base functionality which can be dictated or not that the derived class be required to implement.
Allows for a default operation to be runtime consumed if the derived does not implement or required to implement.
Provides a consistency across derived objects which a base class pointer can utilize without having to have the actual derived; hence allows generic operations on a derived object from a base class reference similar to an Interface in runtime operation.
Interface
Allows a generic pattern of usage as a defacto contract of operation(s).
This usage is can be targetted to the process in hand and allows for the
surgically precise operations for that contract.
Used to help with
factory patterns (its the object returned), mocking of data during
unit tests and the ability to replace an existing class (say from a
factory returning the interface) with a different object and it
doesn't cause any consumer of the factory any pain of refactoring due to the adherence of the interface contract.
Provides a pattern of usage which can be easily understood away from the static of the rest of the class's implementation.
Long story short are they required to get a job done? No.
But if you are into designing systems which will have a lifespan of more than one cycle, the upfront work by said architect will pay off in the long run whether on a team or by an individual.
++Update
I do practice what I preach and when handing off a project to other developers it was nice to say
Look at the interface IProcess which all the primary business classes adhere to. That process defines a system of goals which can help you understand the purpose and the execution of the business logic in a defined way.
While maintaining and adding new functionality to the project the interfaces actually helped me remember the flow and easily add new business logic into the project.
I think if you're not coordinating with others, it does two things
helps keep your from doing weird things to your own code. Imagine
your write a class, and use it in multiple projects. You may evolve
it in one project so that it is unrecognizable from it's cousin in
another project. Having an abstract class or interface makes you
think twice about changing the function signatures.
it gives you flexibility going forward - plenty of classic examples here. Use
the generic form of the thing you're trying to accomplish, and if
you decide you need a different kind later (streamreaders are a
great example, right?) you can more easily implement it later.
Abstract - you can instantiate a child of it, but what is more important, it can has its own non abstract methods and fields.
Interface - more "rough" one in regard of abstract, but in .NET you can have multiple inheritance. So by defining interface you can lead consumer of your interface(s) to subscribe to different contracts(interfaces), so present different "shapes" of specified type.
There are many reasons to use either construct even if you are not coordinating with anyone. The main use is that both actually help express the developper intent, which may help you later figure out why you choose the design you actually chose. They also may allow for further extensibility.
Abstract class allow you to define one common implementation that will be shared across many derived classes while delegating some of the behavior to the child classes. It allows the DRY (don't repeat yourself) principle to avoid having the same code repeated everywhere.
Interfaces expresses that your class implements one specific contract. This has a very useful uses within the framework, among which:
Use of library functionality that necessitate the implementation of some Interface. Examples are IDisposable, IEquatable, IEnumerable...
Use of constraints in generics.
Allow mocking of interfaces (if you do unit testing) whithout having to instanciate a real object.
Use of COM objects
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Interface vs Abstract Class (general OO)
Following are the confusions which I have between interface, abstract and normal class-
I know an abstract class can have a method with no implementation, but what is the harm in declaring method as virtual in base class with some dummy implementation and over ride in derived class. Looking for a practical scenario where answer can be validated?
Why we need an interface when we have abstract class, I know with interface we can have multiple inheritance, and have goggled about various theoretical reasons, looking for practical scenarios where abstract class just cant help and you have to go for interface?
Is having an abstract class and interface is not an overhead?
Practical Examples
I think you are thinking too hard. Its like looking at a fork and a spoon and worrying about when to use when. There is no right answer, just use the one that makes sense to you for what you are doing. Many paths to getting there.
Here is how I use these in the real world (fortune 500 companies etc...)
Interfaces allow you to add as many as you want to a class. They essentially allow you to bind to any contract as much as you want. They are as minimal as you get. An example I used this in was a music encoder service where it just had some simple properties and methods that each encoder type needed. I later added more interfaces as other requirements came into play but I didn't want to break the code for other parts not needing to know or care about the new interface. The encoders were very loosely coupled and could have more than one contract so this made sense for me. The main thing about interfaces is when you use them, don't keep changing them as it defeats the purpose of the interface, rather make a new one. Interfaces are the best for plugin type of development.
Abstract classes are kind of cool because you can have some base implementation already cooked in there but you are essentially saying "but you still have to make this part" as its the responsibility of the class to implement this. This could be for a "save" method for example. Let's say you had some abstract class that handled data in the same way but you had many commonalities in the underlying abstract class that did a lot of work but you had a requirement that each inherited type save its own data in its own format needed for example. This could relate to a networked item that saved things in facebook and twitter or another item that saved things in a database and a file system but the core code always hit a central db to say it was saved with last date modified.
So sorry, I was bored, lots of typing. But that's how I have used them.
There are scenarios where there is no "correct" dummy implementation. In the System.IO.Stream case (an abstract class), for example, what would be a reasonable implementation for Read or Write? Throw? Do nothing? Or the many methods in the System.Xml.XmlReader class - what's a reasonable implementation for a Depth, or Read of an arbitrary XML reader? By making methods in a class abstract (and not virtual) the author of the class is making a statement that anyone who derives from that class must think about what to do for those methods, and the abstract qualifier enforces that.
The practical reason is exactly what you said - multiple inheritance. If IEnumerable<T> were an abstract class, for example, there would be many cases where we'd want to be able to enumerate over the elements of an object but couldn't because the object already inherited from some other class. The more theoretical reason is that interfaces usually define behavior, while classes define a cluster of objects, but for the purposes of your question the practical reason is more, well, practical.
I don't understand your question. Overhead in which sense?
Each has its own purpose. You use an interface when you don't want to define any specific implementation. It only provides an "interface" to an object, that is the methods and properties that make up a public definition. An object can inherit multiple interfaces. A good example of this is the List<> object. List<> implements several interfaces, such as IEnumerable, IList, ICollection, IEnumerable<>, IList<>, and ICollection<>.
Each of these has different methods, and different properties, and are used for different purposes. You could not do this with an abstract class. You can cast a List<> to any of those interfaces and use them without knowing the exact implemenation of them. This is particularly useful with a concept known as Dependency Injection, but has many other uses.
An abstract class is useful when you want to define part of the implementation of an object, but want to leave other parts up to the derived class. One key thing to remember about abstract classes is that they cannot be instantiated by themselves, because they typically lack a complete implementation as well as the fact that abstract keyword tells the complier it can't be instantiated. Yes, you can supply dummy implementations, but why? The whole point is that abstracts are.. well.. abstract.
Imagine you have a class Cat, and this class derives from a class Animal. There is no such thing as an Animal object, it's just a "type". You can't instantiate an Animal because there is no actual Animal in the real world, there are only creatures that are of the type Animal. You can treat different creatures as a common Animal type, but there can't exist a real Animal, it's just a concept.
An Animal would be an abstract type, because the concept of the Animal is abstract. The same is true in software. You might have the concept of an object, such as a Stream. Is it a NetworkStream? A FileStream? A MemoryStream? Stream is just a concept. An interface is also just a concept, but the unlike an abstract class an Interface cannot have any implementation.
So think of an abstract class as a concept with some implementation, and an interface as a concept without any implementation.
What is need of interfaces in c# ? as we are writing abstract method in interfaces. instead of that we can directly implement those methods in class.
Interfaces don't support implementation, so you cannot supply any default implementations as you can with abstract classes. Additionally, interfaces are not restricted to hierarchies, so they are more flexible than abstract classes.
You do not need to use interfaces in C#. They are useful and appropriate in some circumstances, but not in all circumstances. A handy rule of thumb that I use is that if, in your project, you only have one class that implements an interface, you do not need that interface.
Note: one possible counter to this rule of thumb is that you may need to write a second implementing class in the future, which may justify the use of the interface. I do not necessarily agree, as I think a considerable amount of time in programming is wasted anticipating future scenarios which never materialize.
You may want to read up on polymorphism.
For myself, I find a lot of use out of interfaces when I have similar objects but completely different implementations of the same methods.
Additionally, you can implement multiple interfaces but inherit only one abstract class. I find this very useful because my business objects have a better representation.
When writing any N-Tiered application which separates out business logic from the presentation I think you will start to find many uses for interfaces.
Interface is needed exactly as described in books: to define a contract between components. They are one of the best ways to expose certain functionality to other modules while preserving encapsulation.
For example:
1) try, without an interface, to expose some piece of functionality implemented in assembly 'A', to assembly 'B', with no actual implementation visible to assembly 'A'.
2) Even worse - if we consider .NET remoting scenarios, where the server must expose certain
functionality to the client, while the functionality is implemented and hosted on the server side. In this case an assembly is published to a client, where interfaces for the server-hosted classes are defined.
Think of Interfaces as contracts.
You create a contract that a class must follow. For example, if our Dog object must have a Walk method, it's defining class must implement this method.
In order to force every dog class (inherited or not) to implement this method, you must make them adhere to a contract i.e assign an interface which specifies that method.
An interface is a construct that enforces particular classes to follow strict rules of implementation.
The reason for this is that you end up with Dog objects (inherited or not) that now, by default, have a Walk method. This means you can pass these objects as parameters, safe in the knowledge that you can call the Walk method on any Dog class (inherited or not) and it will deffinately be implemented.
If you want to write testable code, you will usually need to employ interfaces. When unit testing, you may have ClassA which depends upon ClassB which Depends upon ClassC etc, but you only want to test ClassA. You certainly don't want to create a ClassC to pass to a ClassB just to instantiate ClassA.
In that case, you make ClassA depend upon IClassB (or some more generic name, most likely that does not imply anything about the ClassB implementation) and mock out IClassB in your tests.
It is all about dependency management for me.
You need interfaces when you want to think of a disparate set of classes as all being the same type of object. Say, for example, you have a set of classes that all read their configuration from a file. One way to handle this is to have all the classes implement the appropriate methods to read a configuration from a file. The trouble with this is that then any code that uses those classes and wants to configure them needs to know about all the different classes so that it can use the methods on them.
Another way is to have them all derive from a single base class. That way any code using the classes need only know about the base class -- it can treat any of the derived classes as the base class and use those methods defined in the base class to do the configuration. This isn't so bad, but it has the major drawback -- since C# doesn't support multiple inheritance -- of limiting your inheritance chain. If you want to be able to have the same sort of ability for some of the classes, but not all of them for a different set of behavior, you're stuck implementing it for all of them anyway. Not good.
The last way is to use interfaces to define the behavior. Any class wanting to implement the behavior need only implement the interface. Any class wanting to use the behavior need only know about the interface and can then use any class that implements it. Classes can implement any interface or even multiple interfaces so you have granular "allocation" of behavior among classes. You can still use base classes and inheritance hierarchies to provide the main behavior of a class in a shared way, but the class is also free to implement other interfaces to give itself more behavior and still retain the convenience of classes that use it to know only about the interface.
Interfaces allow implementers to use their own base class. With abstract classes, implementers are forced to use the given base class to implement the interface even if it actually makes more sense for the implementer to use a project-specific base class.
Interfaces are used to define the behaviour/properties of classes without specifying the implementation. If you begin thinking about classes as fulfilling roles rather than just being a bundle of methods and properties then you can begin assigning multiple roles to classes using interfaces - which is not possible using straight inheritance in C# as you can only inherit from a single class. By making clients of the classes depend on the interfaces (roles) rather than the class itself your code will be more loosely coupled and probably better designed.
Another benefit of this is if you are writing unit tests they are more likely to be focussed on behaviour rather than state. Coding against interfaces makes it very easy to make mock/stub implementations for this type of testing.
Not everything will need interfaces - but most things with behaviour probably (i.e. more than just a bundle of data) should have them IMHO as you'll always have two implementations: a real one in your application and one or more fake ones in your tests.
Interface is a contract that defines the signature of the functionality. So if a class is implementing
a interface it says to the outer world, that it provides specific behavior.
Example if a class is
implementing ‘Idisposable’ interface that means it has a functionality to release unmanaged
resources. Now external objects using this class know that it has contract by which it can dispose
unused unmanaged objects.
As presented by GoF 1st principle: Program to an interface not an implementation.
This helps in many ways. It's easier to change the implementation, it's easier to make the code testable and so on...
Hopes this helps.
I am developing a set of classes that implement a common interface. A consumer of my library shall expect each of these classes to implement a certain set of static functions. Is there anyway that I can decorate these class so that the compiler will catch the case where one of the functions is not implemented.
I know it will eventually be caught when building the consuming code. And I also know how to get around this problem using a kind of factory class.
Just curious to know if there is any syntax/attributes out there for requiring static functions on a class.
Ed Removed the word 'interface' to avoid confusion.
No, there is no language support for this in C#. There are two workarounds that I can think of immediately:
use reflection at runtime; crossed fingers and hope...
use a singleton / default-instance / similar to implement an interface that declares the methods
(update)
Actually, as long as you have unit-testing, the first option isn't actually as bad as you might think if (like me) you come from a strict "static typing" background. The fact is; it works fine in dynamic languages. And indeed, this is exactly how my generic operators code works - it hopes you have the static operators. At runtime, if you don't, it will laugh at you in a suitably mocking tone... but it can't check at compile-time.
No. Basically it sounds like you're after a sort of "static polymorphism". That doesn't exist in C#, although I've suggested a sort of "static interface" notion which could be useful in terms of generics.
One thing you could do is write a simple unit test to verify that all of the types in a particular assembly obey your rules. If other developers will also be implementing the interface, you could put that test code into some common place so that everyone implementing the interface can easily test their own assemblies.
This is a great question and one that I've encountered in my projects.
Some people hold that interfaces and abstract classes exist for polymorphism only, not for forcing types to implement certain methods. Personally, I consider polymorphism a primary use case, and forced implementation a secondary. I do use the forced implementation technique fairly often. Typically, it appears in framework code implementing a template pattern. The base/template class encapsulates some complex idea, and subclasses provide numerous variations by implementing the abstract methods. One pragmatic benefit is that the abstract methods provide guidance to other developers implementing the subclasses. Visual Studio even has the ability to stub the methods out for you. This is especially helpful when a maintenance developer needs to add a new subclass months or years later.
The downside is that there is no specific support for some of these template scenarios in C#. Static methods are one. Another one is constructors; ideally, ISerializable should force the developer to implement the protected serialization constructor.
The easiest approach probably is (as suggested earlier) to use an automated test to check that the static method is implemented on the desired types. Another viable idea already mentioned is to implement a static analysis rule.
A third option is to use an Aspect-Oriented Programming framework such as PostSharp. PostSharp supports compile-time validation of aspects. You can write .NET code that reflects over the assembly at compile time, generating arbitrary warnings and errors. Usually, you do this to validate that an aspect usage is appropriate, but I don't see why you couldn't use it for validating template rules as well.
Unfortunately, no, there's nothing like this built into the language.
While there is no language support for this, you could use a static analysis tool to enforce it. For example, you could write a custom rule for FxCop that detects an attribute or interface implementation on a class and then checks for the existence of certain static methods.
The singleton pattern does not help in all cases. My example is from an actual project of mine. It is not contrived.
I have a class (let's call it "Widget") that inherits from a class in a third-party ORM. If I instantiate a Widget object (therefore creating a row in the db) just to make sure my static methods are declared, I'm making a bigger mess than the one I'm trying to clean up.
If I create this extra object in the data store, I've got to hide it from users, calculations, etc.
I use interfaces in C# to make sure that I implement common features in a set of classes.
Some of the methods that implement these features require instance data to run. I code these methods as instance methods, and use a C# interface to make sure they exist in the class.
Some of these methods do not require instance data, so they are static methods. If I could declare interfaces with static methods, the compiler could check whether or not these methods exist in the class that says it implements the interface.
No, there would be no point in this feature. Interfaces are basically a scaled down form of multiple inheritance. They tell the compiler how to set up the virtual function table so that non-static virtual methods can be called properly in descendant classes. Static methods can't be virtual, hence, there's no point in using interfaces for them.
The approach that gets you closer to what you need is a singleton, as Marc Gravell suggested.
Interfaces, among other things, let you provide some level of abstraction to your classes so you can use a given API regardless of the type that implements it. However, since you DO need to know the type of a static class in order to use it, why would you want to enforce that class to implement a set of functions?
Maybe you could use a custom attribute like [ImplementsXXXInterface] and provide some run time checking to ensure that classes with this attribute actually implement the interface you need?
If you're just after getting those compiler errors, consider this setup:
Define the methods in an interface.
Declare the methods with abstract.
Implement the public static methods, and have the abstract method overrides simply call the static methods.
It's a little bit of extra code, but you'll know when someone isn't implementing a required method.